# LTspice UVLO excercise

Electrical Engineering Asked by Paumdrdo on November 22, 2020

How do I properly set this up in LTspice to determine Vin during rising and falling? I kind of a loss on how to answer this any help would be appreciated.

I agree that this should be done manually, which is quite straightforward, however Here is a simulation using LTspice:

The behavioral current source equation

I = 2.5E-6*(0.5 + 0.5tanh( 100(1.22-V(V_control) )))

simply provides a current source of 2.5uA if the voltage at V_control is < 1.22V and 0uA if it is greater than 1.22V. The hyperbolic tangent function provides a 'smooth' output that is differentiable.

Answered by Spehro Pefhany on November 22, 2020

You can consider the two cases separately.

The "Vin falling" case will just be a simple resistive voltage divider. Given $$V_{uvlo} = 1.22text{V}$$, can you solve for $$V_{in}$$?

The "Vin rising" case will have the current source connected to $$V_{uvlo}$$. The question doesn't specify whether current should be going into or out from the node. You should be able to solve this case using KCL. I suggest you solve for both cases (current in and current out) and see which one causes the circuit to have the right type of hysteresis. Have you learned what hysteresis is?

Answered by Justin on November 22, 2020